A nephelometric turbidimeter determines the concentration of solid or other particles suspended in a fluid, which can generally be a liquid, a gas, or a mixture of liquid and gas. U.S. Pat. No. 8,724,107 B2 describes a nephelometric turbidimeter with a cylindrical turbidimeter vial comprising a transparent and flat bottom inlet window through which a measurement light beam axially enters the vial interior. An axial middle section of the vial cylinder body defines a transparent outlet window through which the light scattered by the suspended solid particles radially exits the vial interior. The cylindrical outlet window is radially surrounded by an annular light collecting means which optically cooperates with a light sensor which detects the scattered and collected light. The primary light signal, which is only caused by the light scattered within the relevant fluid volume, is relatively small so that any disturbing light causing a secondary signal should, if possible, be avoided.
The vial body is provided with a cylindrical optical shielding at the cylinder body, the shielding being axially located between the bottom inlet window and the circular outlet window. This shielding provides that light scattered by a particle layer at the bottom inlet window of the vial cannot directly irradiate the light collecting means. No shielding is provided in the upper portion of the vial axially beyond the circular outlet window because the scattered light coming from the bottom inlet window is totally reflected by and at the boundary between the vial body and the surrounding atmosphere.
The vial body has a top opening which is closed by a vial cap which can be a part of the turbidimeter to provide a fluid-tight vial interior. The measurement light beam can be reflected downwardly at an interior wall of the vial cap, and can thereby be directed directly or indirectly to the light collecting means and/or the optical turbidity sensor, and can thereby generate a secondary signal. The vial in a laboratory turbidimeter is normally not completely filled with a liquid sample so that the measurement light beam can also be reflected downwardly at the boundary layer of liquid and air.
In drinking water applications, the measurement light beam's intensity is only minimally reduced by scattering in the fluid sample so that the reflected measurement beam causes secondary signals being many times more intense than the primary signal.
The vial in a laboratory turbidimeter is exchanged for every measurement, so that the vial is manually taken out of a vial chamber of the turbidimeter. If the vial is put down on a hard surface, the outside of the bottom inlet window can be scratched by the hard surface. A scratch can cause strong optical fractions of the measurement light beam passing the scratch at the bottom inlet window, thereby causing intense secondary signals.
Since the vial is exchanged manually in a laboratory turbidimeter, fingerprints causing optical fraction and scattering also cannot be avoided. The fingerprints can thereby also cause secondary signals and falsify the result of the turbidity measurement.